This invention relates generally to integrated circuits and more particularly to memory cells for programmable read-only memories (PROMs) and the like.
Field programmable gate arrays include a large number of logic elements, such as AND gates and OR gates, which can be selectively coupled together by means of fuses or anti-fuses to perform user designed functions. An unprogrammed fuse-type gate array is programmed by selectively blowing fuses within the device, while an unprogrammed anti-fuse type gate array is programmed by causing selected anti-fuses to become conductive.
There are many types of PROMs including standard, write-once PROMs, erasable programmable read-only memories (EPROMS), electrically erasable programmable read-only memories (EEPROMS) etc. A PROM usually comprises an array of memory cells arranged in rows and columns which can be programmed to store user data. PROMs are typically fuse-type devices.
Fuses for field programmable gate arrays, PROMs and the like are typically made from a titanium-tungsten (TiW) alloy and are shaped somewhat like a bow-tie having a narrow, central neck and wide ends. The neck of the fuse is typically about 2 microns wide, while the ends of the fuse are typically about 6 microns wide. When a sufficiently high voltage (usually on the order of 10 volts D.C.) is applied to the fuse, the current flowing through the fuse will cause it to heat-up and will eventually melt the fuse at its neck, thereby "blowing" the fuse.
Anti-fuses include a material which initially has a high resistance but which can be converted into a low resistance material by the application of a programming voltage. For example, amorphous silicon, which has an intrinsic resistivity of approximately 1 megohms-cm, can be fashioned into 1 micron wide link vias having a resistance of approximately 1-2 gigohms. These link vias can then be melted and re-crystallized by the application of a programming voltage in the range of 10-12 volts D.C. to form link vias having a resistance less than 200 ohms. These low resistance vias can couple together logic elements of a field programmable gate array so that the gate array will perform user-desired functions, or can serve as memory cells of a PROM. Herein, the term "fuses" is used generically to refer both to fuses (a material which initially has a low resistance but which can be converted into a high resistance material by the application of a programming voltage) and to anti-fuses (material which initially has a high resistance but which can be converted into a low resistance material by the application of a programming voltage).
Anti-fuse structures have the very desirable feature of being small in size. For example, a TiW fuse with a 2 micron neck and 6 micron end widths permits approximately 4,000 fuses to be provided on a typical device. In contrast, a 1 or 1.2 micron diameter anti-fuse via permits 80,000-100,000 fuses to be provided on a single device. Therefore, anti-fuses have the potentiality of providing vastly greater numbers of interconnections or of storing much greater amounts of information than devices using fuse technology. However, fuses in electronic devices are much more prevalent today than anti-fuses because they are easier to manufacture and have a better record of reliability. Improved manufacturing techniques have overcome several problems which formerly limited the use of anti-fuse structures.
However, in the prior art, the anti-fuse was typically built between metal one and metal two layers. This requires several additional masking steps and etch steps. For example, an extra metal layer of Titanium Tungsten, a link via, link oxide and link spacer are all required. Additional layers increase the topology and leads to metal step coverage problems. Also, defect density is increased as rigorous cleansing is not possible after metal deposition. In addition, in prior art methods, programming voltage varies a lot due to the non-conformal nature of amorphous silicon film deposited by plasma enhanced chemical vapor deposition (PECVD) techniques. See, for example, "Very High Speed FPGAs 1992 Data Book", from QuickLogic Corporation, pp. 1-1 through 2-4.